This invention relates to coding of digital signals. More particularly, this invention relates to enhancement of differential coding of signals that improves the noise immunity of the coded signals.
In principle, it is possible to transmit analog video signals (image signals) in digital form, which is typically obtained by sampling and linearly quantizing the analog signals. When image signals are digitized and linearly quantized, a transmission rate of about 100 Mbits per second is necessary for images derived from standard TV signals. For HDTV, a much higher transmission rate would be required.
In order to reduce this rate, various coding schemes have been studied. One is the so-called "differential pulse code modulation" (DPCM) coding approach. By this method, the value of a particular pixel at any moment is predicted on the basis of values of pixels which have been already coded. The necessary number of bits is reduced by coding the difference (error) between the predicted value and the value of the particular pixel at that moment. In the above-mentioned related application, the encoding process is similar in that video signals are also encoded differentially. The code developed for a pixel group is a function of the difference between the pixel group and a previous pixel group.
The disclosed encoding method has a problem in that it is unforgiving of errors. More specifically, since the encoding process needs information about past signals, both the encoder and the decoder include feedback loops which provide the needed past signal information. As long as no errors are introduced between the encoder and the decoder, the two loops track each other and the decoder output faithfully reproduces the encoder's input (within the encoder's quantizing error). Alas, when an error is introduced in the decoder, it has no way of knowing the presence of this error. What is more disturbing, however, is that the decoder's feedback loop perpetuates this error. That is, a noise spike can be easily tolerated when it occurs momentarily, but if it persists, then it is no longer a noise spike.
The problem of error perpetuation is solved in the '475 patent by causing a changing, rather than a fixed, fraction of the input signal to leak through to the encoder. The fraction leaked, is sensitive to the characteristics of the signal. In one embodiment, the fraction leaked is fixed for a frame in accordance with a chosen characteristic of the frame signal. In another embodiment, the fraction leaked is set in accordance with one function when a chosen characteristic of the frame signal exceeds a given level, and follows another function when the chosen characteristic does not exceed the chosen level. In a still another embodiment, the fraction leaked is set to one of two levels, based on a chosen characteristic of the frame signal.